Films made of propylene copolymers or terpolymers are known in the art.
Propylene copolymers or terpolymers are used because, with respect to propylene homopolymers, are characterized by a better impact, lower rigidity and better transparency. In some cases however, it is difficult to find the acceptable balance between those properties, particularly when properties contrasting to each other are desired. When a certain softness is desired, for example, it is commonly obtained in the presence of high amount of xylene soluble fractions that make them unsuitable for food contact applications.
U.S. Pat. No. 6,221,984 discloses random copolymers of propylene with ethylene and at least one C4-C10 alpha-olefin and a process for preparing such random copolymers, which can be used in films, fibers or moldings. In particular, the terpolymers obtained by the process disclosed in this specification are particularly suitable for food packaging films because of their low proportions of xylene-soluble polymer particles (Examples 1-3) but are not suited for applications requiring a certain degree of softness.
On the other hand, when the xylene soluble fraction is increased (comparative examples 1 and 2) and softness is increased, the sealing initiation temperature and the optical properties become unsatisfactory.
WO 03/037981 discloses pipes made from at least a polypropylene composition obtained by a process carried out in a reactor comprising two interconnected polymerization zones.
Said process provides polypropylene compositions with high stiffness and impact resistance particularly suitable for pipes. When the propylene composition is a propylene-ethylene-butene-1 copolymer, the flexural modulus is higher than 700 MPa.
The propylene composition disclosed by WO 03/037981 cannot be used in the preparation of films when a certain degree of softness is required.
Furthermore, WO 98/58971 discloses a process for producing terpolymers of propylene, ethylene and other alpha-olefins comprising slurry and gas phase reactors connected together. The process comprises using a combination of two or more reactors connected in a cascade for producing a polymer product exhibiting a ratio of ethylene-to-butene less than 0.3. Such comonomer distribution gives a material having low soluble content in hexane and good optical properties.
The drawback related to the above mentioned terpolymer product is the too low melting temperature and the consequent narrow processability window. In fact, with respect to a given comonomer units content, the higher is the melting temperature of the copolymer, the wider is its processability window.
WO 2009/019169 relates to a propylene/ethylene/1-butene terpolymer prepared in a gas phase reactor comprising two interconnected polymerization zones. Said terpolymer has among other features the ration between ethylene amount (wt %) and 1-butene amount (wt %) ranging from 0.1 to 0.8 and a xylene soluble fraction at 25° C. higher than 9 wt %.
The applicant found that by lowering the ethylene/1-butene ratio (wt %/wt %) and the xylene soluble fraction it is possible to obtain a material having a low SIT, better transparency and lower hexane extractables. Thus making this material fit for films especially in food packaging.